Fusible pulverulent material (hereinafter sometimes powder) coating systems are known. An illustrative, but by no means exhaustive, list of such systems includes U.S. Pat. Nos. 8,431,180; 8,420,180; 8,381,381; 8,372,478; 8,356,574; 8,353,469; 8,333,570; 8,329,781; 8,329,258; 8,312,835; 8,309,232; 8,292,197; 8,282,753; 8,268,381; 8,166,912; 8,132,743; 8,123,147; 8,113,140; 7,997,963; 7,968,206; 7,878,420; 7,837,130; 5,399,198; and, 5,131,350. The disclosures of these references are hereby incorporated herein by reference. This listing is not intended to be a representation that a complete search of all relevant art has been made, or that no more pertinent art than that listed exists, or that the listed art is material to patentability. Nor should any such representation be inferred.
Powder pipe coating systems typically include booths containing several powder dispensing devices (hereinafter sometimes powder guns, or guns). The number of guns depends, among other things, upon the quantity of powder which must be dispensed per unit time (per minute, for example) to provide a desired powder film build on the pipe as the pipe passes through the booth. Typically, lengths of pipe are heated to, for example, about 450° F. (about 232° C.) prior to passage through the booth. The ends of the pipe sections are masked to permit later welding of powder coated pipe sections to one another. The pipe sections are passed through the booth as powder is dispensed by the guns onto them. The powder essentially fuses on contact with the hot surface of the sections of pipe. The pipe is then typically water quenched to bring its temperature below the powder's fusion temperature, stacked and permitted to cool, completing the fusion of the protective coating to the outer surface of the pipe.
Typically, the oversprayed powder exits the booth and passes into ductwork coupled to, for example, a cyclone or baghouse for removal of the oversprayed powder from the airstream and, where possible, recovery of the oversprayed powder for recirculation. However, the presence of the powder in the ductwork constitutes a hazard, and, as a result, in order to comply with, for example, Occupational Safety and Health Act requirements, the ductwork/collection vessel is typically required to have dust/explosion suppression systems such as, for example, those available from Fike Corporation or Fenwal Protection Systems.
A better solution would be to permit the overspray-laden airstream to pass through a filter enclosure attached to, or closely associated with, the booth, that is, without intermediate ductwork between the booth and a baghouse or cyclone, so that the powder could be removed from the airstream in the immediate vicinity. There would then be no need for the ductwork and its associated components, such as, for example, the dust/explosion suppression system. Ideally, the filter enclosure could be integrated into the booth. The problem with such a facility, however, is that any oversprayed powder remains hot as the overspray exits the spray area, typically hotter than its melting temperature, from its passage near the pipe. Thus, the powder impacting against the filter elements is prone to fuse to the elements, fairly quickly plugging them and rendering the filter inoperable.